Full bridge inverters employing switching elements are well known in the art. Typically, such circuits include four bipolar transistors of like conductivity type. A first pair of the transistors is connected in a first series circuit between a DC power source and a bridge apex that is connected to a load so that current flows in a first direction through the apex during a first time interval while both transistors of the first pair are forward biased. A second pair of the transistors is connected in a second series circuit between the DC source and the apex so that current flows in a second direction through the apex during a second time interval while the transistors of the second pair are forward biased. Forward biasing of the transistors of the first and second pairs occurs during alternate half cycles of a time reference AC source, typically coupled in out of phase relationship between the base and emitters of the transistors of the two pairs. Typical of prior art patents disclosing such circuits are Bright et al, U.S. Pat. No. 2,821,639, and Norton, U.S. Pat. No. 2,872,582.
Because the switching transistors of one pair are essentially in series while they are forward biased into conduction, it is advantageous for the switching transition times of these transistors to be the same; it is particularly advantageous for the cut-off times of the transistors of each pair to be the same. The necessity for simultaneous transition times of the switching transistors of each pair can be obviated if the power ratings of the transistors are very large. However, it is not advantageous to provide transistors with excessively large power ratings becuse of cost, heat dissipation and size.
If transistors with low power ratings are used, they must have simultaneous conduction and cut-off times for numerous reasons. In particular, if the low power transistors are simultaneously activated into the forward biased condition, turn-on losses of both transistors of one pair are equalized, to provide equal heat losses in both transistors and symmetrical heat sinking. Simultaneous turn-off times of the two transistors distributes turn-off losses equally between the two transistors of a pair. If one transistor turns off appreciably before the other, there is equal power dissipation. The transistor which turns off first must sustain a full load current while it is in a cut-off condition, as well as the relatively high inductive voltage which results from a collapsing magnetic field of a transformer in the apex. The slower transistor has essentially no dissipation during turn off because the current through it is interrupted by the turn off of the faster transistor. Equalizing the turn-on and turn-off times of the transistors of a single pair greatly reduces switching noise and transients. If both transistors of a pair are simultaneously turned on and turned off, the open circuit emitter collector voltage of each transistor can materially be reduced.
In the prior art, it is difficult to obtain inexpensive, off-the-shelf transistors that are matched to have characteristics so that they simultaneously are activated into a conducting state and deactivated into a cut-off state. Of course, matched transistor pairs, having these desired characteristics exist, but the price of such matched pairs is generally significantly higher than the price of unmatched, off-the-shelf transistors.
In view of the foregoing, it is an object of the present invention to provide a new and improved full bridge inverter having switching elements that are simultaneously turned on and turned off in pairs.
A further object of the invention is to provide a new and improved full bridge inverter wherein conventional, off-the-shelf, relatively low power bipolar transistors can be employed as switching elements.
A further object of the invention is to provide a full bridge switching inverter employing switching transistors having relatively low open circuit emitter collector voltage ratings.
Still another object of the invention is to provide a new and improved full bridge switching inverter wherein there is a substantial reduction of noise and transients associated with the switching elements being activated into and out of a conducting state.